Nonvolatile semiconductor memories which are known, for example, as FLASH memory, EPROM, EEPROM, FPGAs and the like, are becoming more and more successful in many fields such as, e.g. dataprocessing, telecommunications, entertainment, electronics and security technology since they can store data in a very small space over a long period and without using a supply voltage.
In this connection, there is a multiplicity of different storage element types, the present invention relating to, in particular, a so-called split-gate storage cell.
FIG. 1 shows a simplified sectional view of such a conventional split-gate storage element.
According to FIG. 1, a channel region located in a semiconductor substrate 1 between a source region S and a drain region D is split into a first part-section I and a second part-section II, a control layer 5 being formed directly above the channel region and being separated from this by a first insulating layer 2A in the first part-section whereas it is located only indirectly above the channel region or semiconductor substrate 1, respectively, in a second part-section II. To achieve the desired charge-storing characteristics, instead, a charge storage layer 3 is formed above the channel region or semiconductor substrate 1, respectively, and insulated from the latter by a second insulating layer or tunnel layer 2B in the second part-section II.
To implement a so-called source side charge carrier injection (SSI—source side injection), the split-gate storage element according to FIG. 1 also has a programming layer 6 which is essentially formed on the surface of the charge storage layer 3 and separated or insulated from the latter by a third insulating layer 4.
For programming or injecting charge carriers into the charge storage layer 3, a programming electrode PG connected to the programming layer 6, a control electrode CG connected to the control layer 5, a source line SL connected to the source region S and a bit line BL connected to the drain region D are connected in such a manner that at the transition between the first and second part-section I and II, an electrical field is built up in the channel area which is of such an amplitude that, due to the high potential gradient present, electrons coming from the source region S are accelerated in such a manner that they are injected in to the charge storage layer. Such programming under SSI (source side injection) conditions extends the life of storage elements due to the reduced stress on the insulating layers compared with the excessive electrical fields required for a drain-side charge carrier injection. In addition, programming under SSI conditions is much more efficient than a drain-side charge carrier injection as a result of which, in particular, the time required for such programming is reduced or, with the same programming time, the channel current and thus the power consumption can be reduced. In particular, however, the operating voltages can be significantly reduced in the case of storage elements with source side charge carrier injection.
A disadvantageous factor in such storage elements with source side charge carrier injection is, however, too much greater design expenditure resulting, in particular, from the three separate control layers—charge storage layer 3, programming layer 6 and control layer 5. It is particularly due to the additional programming layer 6 and a lack of self-adjustment that high integration densities can only be implemented to a limited extent for such split-gate storage elements.
FIG. 2 shows a simplified sectional view of a further nonvolatile storage element, where essentially a so-called CHE (channel hot electron) charge carrier injection takes place at the drain by means of hot charge carriers from the channel.
According to FIG. 2, such a nonvolatile storage element consists of a semiconductor substrate 1 in which a source region S, a drain region D and an intermediate channel region are formed, a charged storage layer 3 being formed by a first insulating layer 2 on the channel region, separated from the latter, and again being suitable for storing charge carriers. On the surface of the charge storage layer 3 is again a control layer 5 which is separated from the charge storage layer 3 by a further insulating layer 4.
In contrast to the split-gate storage cell with source-side charge carrier injection, described above, this nonvolatile storage cell only needs three contact connections, namely a control electrode CG or wordline WL to connect the control layer 5 and a source line SL for connecting the source region S and a bitline BL for connecting the drain region. As a result, the structure and thus also the production of such a conventional storage cell is considerably simplified and an increased density of integration can be achieved particularly due to the lack of a contact connection for a programming layer.
A disadvantageous factor in such a nonvolatile storage cell is, however, the use of necessary and high operating voltages in order to achieve a CHE (channel hot electron) channel injection by means of hot charge carriers. In particular, these high drain and gate voltages are the result of the effort to shorten the programming time which is why programming essentially takes place in the vicinity of the breakdown voltages. In consequence, such high voltages for implementing a charge carrier injection under CHE conditions need additional voltage supply circuits and an extremely high stress on the insulating layers provided.
FIG. 3 shows a simplified sectional view of a further conventional nonvolatile semiconductor storage element for storing two bits such as is known, for example, from the printed document U.S. Pat. No. 6,366,500.
According to FIG. 3, a source region S and a drain region D with an intermediate channel region is again formed in a semiconductor substrate 1, which channel region exhibits a first part-section I and two second part-sections IIA and IIB at the source and at the drain. On the surface of the semiconductor substrate 1 or the channel region, respectively, there is again a first insulating layer as gate dielectric or as tunnel dielectric, a control layer 5 being formed on the surface of the first insulating layer 2 in the first part section I and a drain side charge storage layer 3A and a source side charge storage layer 3B, which have doped polysilicon as electrically conductive floating gates, being formed in each case in the two second part-sections IIA and IIB of the channel region.
To implement the abovementioned source side charge carrier injection or SSI condition, respective drainside and source side programming layers 6A and 6B, which are insulated or separated from the respective charge storage layers 3A and 3B by a further insulating layer 4A and 4B, are located at the charge storage layers 3A and 3B.
Although this provides a so-called dual-bit split-gate storage cell with source side charge injection, the complexity and space requirement are again increased due to the programming layers 6A and 6B used.
FIG. 4 shows a simplified sectional view of a further conventional dual-bit split-gate storage cell, where identical reference symbols again describe identical or corresponding elements as in FIGS. 1 to 3 and a repeated description is omitted in the text which follows.
According to FIG. 4, it is again possible to store two storage states, i.e. two bits, at the source and at the drain in a charge storage layer 3A and 3B, but such a storage element uses an electrically nonconductive silicon nitride layer as the charge storage layer. Such a split-gate storage element as is known, for example, from the printed document U.S. Pat. No. 5,408,115, again results in advantageous or low programming voltages due to the adjustable SSI condition, in which case the complexity and the space required for achieving such a storage element are again very high.